Low-electrostatically-charging granular polytetrafluorethylene powder and preparation process of same

ABSTRACT

To provide a granular PTFE powder having a low electrostatic charging property even after drying. A process for preparing a low-electrostatically-charging granular polytetrafluoroethylene powder by contacting a polar group-containing organic compound having an electrostatic charging-preventing ability when substantially dry to a granular polytetrafluoroethylene powder and then drying the granular powder while the polar group-containing organic compound is kept remaining in the granular powder.

TECHNICAL FIELD

The present invention relates to a granular polytetrafluoroethylene(PTFE) powder having a low-electrostatically-charging property and apreparation process thereof.

BACKGROUND ART

PTFE, particularly PTFE prepared by suspension polymerization is, afteronce pulverized to an average particle size of not more than 100 μm,granulated by various granulation methods such as a dry granulationmethod, a method of granulation in the state of emulsion, a granulationmethod in the state of aqueous dispersion and a method of granulation bywetting and if necessary, subjected to shaping into a granular powderhaving an average particle size of about 100 to 600 μm. The granularpowder after the shaping is used as a molding powder for powder molding.

However since PTFE has electrically insulating property, it is easilycharged when dry, its powder flowability is lowered and in addition, itsticks to a transportation pipe and a molding die. Once PTFE powdersticks thereto, complicated steps are required when changing to otherproduct, and it is impossible to change from PTFE to other productparticularly when PTFE is transported through the pipe.

Therefore it is proposed that after granulation, a granular powder istreated with an aqueous solution of ion-dissociative substance(electrolyte) such as an inorganic acid or inorganic salt toelectrically neutralize static electricity (JP-B-53-13230). However inthat method, though the charged granular PTFE powder can be onceneutralized electrically, there is no effect on prevention ofelectrostatic re-charging caused by a friction at transporting andmolding the powder after drying since the electrolyte is used.

The present inventors have found that a granular PTFE powder granulatedin the presence of a surfactant has unexpectedlylow-electrostatically-charging property, and as a result of furtherinvestigations, have found that a polar group of the surfactant has afunction to prevent electrostatic charging and exhibits an electrostaticcharging-preventing ability at the time when the powder is substantiallydry, concretely when it is transported and is molded after drying. Thusthe present invention was completed.

An object of the present invention is to provide alow-electrostatically-charging granular PTFE powder which can maintainan electrostatic charging-preventing ability even after driedsubstantially, and a preparation process thereof.

DISCLOSURE OF THE INVENTION

The present invention relates to a process for preparing alow-electrostatically-charging granular PTFE powder by contacting agranular PTFE powder to a polar group-containing organic compound havingan electrostatic charging-preventing ability after the powder is driedsubstantially, and then drying the granular PTFE powder with the polargroup-containing organic compound being left contained in the powder.

In that preparation process, it is preferable that after the polargroup-containing organic compound in the form of an aqueous solution iscontacted to the granular PTFE powder, the granular powder is driedwithout washing.

The effect of the present invention can be exhibited particularly whenthe granular PTFE powder contains no filler or when even if the filleris contained, the filler is an electrically insulating filler.

As a polar group-containing organic compound which has anelectrostatically charging-preventing ability after dried substantially,preferred is a surfactant, particularly an anionic surfactant ornonionic surfactant. In case where the surfactant is used in the form ofan aqueous solution, it is preferable that a content thereof is 1 to30,000 mg/liter, particularly 25 to 10,000 mg/liter.

Further the present invention relates to the granular PTFE powdercontaining a polar group-containing organic compound in an amount of 10to 300 ppm and having an electrostatic charge of not more than 50 V,preferably not more than 10 V.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of an apparatus used for determiningflowability of the granular powder in examples.

BEST MODE FOR CARRYING OUT THE INVENTION

The granulation method of the granular PTFE powder which can be used inthe preparation process of the present invention is not limited, and thegranular PTFE powder can be prepared by various granulation methods(JP-B-44-22619, JP-B-44-22620, JP-B-60-21694, JP-A-3-259925,JP-A-9-52955, WO97/15611 and WO97/11111). Also the preparation processof the present invention is effectively applied particularly to agranular PTFE powder containing no filler with the electrostatic chargeproblem and a granular PTFE powder containing an electrically insulatingfiller. With respect to a granular PTFE powder prepared by a granulationmethod using a surfactant at granulating, for example, a granulationmethod in the state of emulsion or PTFE containing an electricallyconductive filler, an amount of a polar group-containing organiccompound having electrostatic charging-preventing ability (hereinafterin some cases referred to simply as “electrostatic charging-preventingcompound”) may be decreased.

JP-A-3-255133 discloses a method of removing a hydrophobic fillersticking to a surface of a filler-containing granular PTFE powder bystirring and washing with an aqueous solution of a surfactant. Howeversince this method intends to wash the filler on the granular PTFE powderwith an aqueous solution of a surfactant, an electrostaticcharging-preventing property seems not improved because the surfactantis removed by washing with water after treatment. Further that patentpublication discloses neither lowering of an electrostatic chargingproperty nor electrical properties of the granular powder aftertreatment.

As PTFE used in the present invention, there may be usedtetrafluoroethylene homopolymer or a modified PTFE containing acopolymerizable component in an amount of not more than 5% in a rangenot losing non-melt-processability. The granular powder may be subjectedto the treatment of the present invention for lowering electrostaticcharge as it is after the granulation or after a step for shaping of thegranular powder. The granular powder may be subjected to such treatmentafter once dried or in the form of being wetted with water.

Filler-containing granular PTFE powders to be used are those prepared byknown processes. As mentioned above, the present invention is appliedeffectively in case where a powder contains an electrically insulatingfiller which involves a problem with electrostatically chargingproperty. Examples of the electrically insulating filler are, forinstance, inorganic fillers such as glass fiber and powder, molybdenumdisulfide powder and fluorinated mica powder; organic fillers such as awholly aromatic polyester resin powder, polyimide powder, polyphenylenesulfide powder and tetrafluoroethylene-perfluoro(alkyl vinyl ether)copolymer powder; and the like. Examples of the electrically conductivefiller are, for instance, metal fibers or metal powders such as bronzepowder, gold powder, silver powder and stainless steel powder, cokepowder, carbon fiber, and the like. Among them, it is preferable thathydrophilic fillers such as glass fiber are previously surface-treatedwith a surface-treating agent such as aminosilane to be hydrophobic.

The electrostatic charging-preventing compound may be a polargroup-containing organic compound having an electrostaticcharging-preventing ability when substantially dry. Examples of thepreferred electrostatic charging-preventing compound are a surfactant,polymethyl methacrylate, polyurethane, and the like. Among them, asurfactant having no effect on physical properties of afluorine-containing resin is preferable.

As the surfactant, any of anionic, cationic and nonionic surfactants canbe used. In the present invention, since a desired effect can beobtained by letting the surfactant remain in the granular powder,anionic and nonionic surfactants, particularly a nonionic surfactant arepreferable from the viewpoint of being easily kept remaining in afluorine-containing resin.

As the anionic surfactant, there can be used known ones, for example,higher fatty acid and its salt, alkyl sulfate, alkyl sulfonate, alkylaryl sulfonate, alkyl phosphoric acid ester, and the like. Particularlypreferable anionic surfactants are a sulfate of higher alkyl alcohol,for example, sodium lauryl sulfate or a fluorine-containing sulfonicacid type- or carboxylic acid type-anionic surfactant having afluoroalkyl group or chlorofluoroalkyl group. The typical compoundsthereof are those represented by the formula (V):

X(CF₂CF₂)_(n)(CH₂)_(m)A  (V)

or the formula (VI):

X(CF₂CFCl)_(n)(CH₂)_(m)A  (VI)

wherein X is hydrogen atom, fluorine atom or chlorine atom, n is aninteger of 3 to 10, m is 0 or an integer of 1 to 4, A is carboxyl group,sulfonic acid group or an alkali metal or ammonium residue thereof.

From the viewpoint that coloration of a molded article is difficult toarise when it is sintered by heating, an anionic surfactant having aperfluoroalkyl group or perchlorofluoroalkyl group as a hydrophobicgroup is preferable.

Examples of the polar group of the anionic surfactant are a sulfonicacid group, sulfuric acid ester group, phosphoric acid ester group, andthe like.

Examples of the above-mentioned nonionic surfactant are, for instance,polyoxyethylamine oxides, alkylamine oxides, polyoxyethylene alkylethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene fatty acidesters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acidesters, glycerine esters, polyoxyethylene alkylamine, segmentedpolyalkylene glycols having a hydrophobic segment comprisingpoly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilicsegment comprising poly(oxyethylene) unit, the derivatives thereof, andthe like.

More particularly, examples of the polyoxyethylamine oxides aredimethyloxyethylamine oxide, and the like.

Examples of the alkylamine oxides are dimethyllaurylamine oxide,dimethyloleylamine oxide, and the like.

Examples of the polyoxyethylene alkyl ethers are polyoxyethylene laurylether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether,polyoxyethylene oleyl ether, polyoxyethylene behenyl ether, and thelike.

Examples of the polyoxyethylene alkyl phenyl ethers are polyoxyethylenenonyl phenyl ether, polyoxyethylene octyl phenyl ether, and the like.

Examples of the polyoxyethylene fatty acid esters are polyoxyethylenemonolauric acid ester, polyoxyethylene monooleic acid ester,polyoxyethylene monostearic acid ester, and the like.

Examples of the sorbitan fatty acid esters are sorbitan monolauric acidester, sorbitan monopalmitic acid ester, sorbitan monostearic acidester, sorbitan monooleic acid ester, and the like.

Examples of the polyoxyethylene sorbitan fatty acid esters arepolyoxyethylene sorbitan monolauric acid ester, polyoxyethylene sorbitanmonopalmitic acid ester, polyoxyethylene sorbitan monostearic acidester, and the like.

Examples of the glycerine esters are monomyristic acid glyceryl,monostearic acid glyceryl, monooleic acid glyceryl, and the like.

Examples of the derivatives of the above surfactants are, for instance,polyoxyethylene alkyl phenyl-formaldehyde condensate, polyoxyethylenealkyl ether phosphate, and the like.

As the segmented polyalkylene glycols having a hydrophobic segment andhydrophilic segment, preferable are, for example, those represented bythe formula (IV):

H—(OCH₂CH₂)_(q)—(A)_(p)—(CH₂CH₂O)_(q)—H  (IV)

wherein A is

p is an integer of 5 to 200, q is an integer of 2 to 400. From theviewpoint of being easily adsorbed in the PTFE resin, it is preferablethat p is from 15 to 40 and q is from 7 to 100.

Among them, preferable are amine oxides, polyoxyethylene alkyl phenylethers and segmented polyalkylene glycols. Further preferable arepolyoxyethylamine oxide and

From the viewpoint that coloration of a molded article is difficult toarise when it is sintered by heating, a nonionic surfactant having ahydrophobic segment comprising poly(oxyalkylene) unit having 3 or 4carbon atoms and a hydrophilic segment comprising poly(oxyethylene) unitis preferable.

Examples of the polar group of the nonionic surfactant are apolyoxyethylene moiety, polyoxypropylene moiety, polyoxybutylene moiety,and the like.

Examples of the cationic surfactant are, for instance,polyvinylbenzyltrimethylammonium chloride, a quaternary compound ofpolydimethylaminoethyl methacrylate, polydiallyldimethylammoniumchloride, and the like.

Example of the polar group of the cationic surfactant is an ammoniumsalt moiety.

As a method of contacting an electrostatic charging-preventing compoundto a granular PTFE powder, there are:

(1) a method of pouring the granular PTFE powder into an aqueoussolution of electrostatic charging-preventing compound,

(2) a method of spraying an aqueous solution of electrostaticcharging-preventing compound to the granular PTFE powder,

(3) a method of pouring an electrostatic charging-preventing compoundinto a granulation tank immediately after granulation (shaping) of thegranular PTFE powder,

and the like method. Among them, from the viewpoint of production cost,the method (3) is preferable.

By making such a contact, the electrostatic charging-preventing compoundis stuck to the granular PTFE powder. The granular PTFE powder is driedwith the electrostatic charging-preventing compound being stuck to andremaining on the granular PTFE powder, if necessary, after separating anexcess aqueous solution of the electrostatic charging-preventingcompound.

In the present invention, since the electrostatic charging-preventingcompound is required to be kept remaining on the granular PTFE powder,it is not particularly necessary to wash the powder with water, etc.after the contact.

It is preferable that an amount of the electrostatic charging-preventingcompound remaining on the granular PTFE powder is from 10 to 300 ppm,particularly from 20 to 150 ppm on the basis of the granular PTFEpowder. When the remaining amount is in the above-mentioned range, it ispossible to decrease an electrostatic charge of the granular PTFE powderto not more than 50 V, preferably not more than 30 V, particularly notmore than 10 V.

According to the preparation process of the present invention, it ispossible to prevent the granular PTFE powder from beingelectrostatically charged even after dried substantially withoutlowering inherent properties of the granular PTFE powder, from loweringpowder flowability and from sticking to a die for molding.

The preparation process of the present invention is then explained bymeans of examples, but the present invention is not limited to them.

The properties evaluated in examples and comparative examples aredetermined by the following methods.

Apparent density: Measured according to JIS K 6891-5.3.

Average particle size after pulverization (Primary particle size):

Wet sieve method: JIS standard sieves of 20 mesh (sieve opening: 840μm), 250 mesh (sieve opening: 62 μm), 270 mesh (sieve opening: 53 μm),325 mesh (sieve opening: 44 μm) and 400 mesh (sieve opening: 37 μm) areused. First, the 20 mesh sieve is placed on the 250 mesh sieve, and 5 gof a sample powder is put on the 20 mesh sieve. By spraying carbontetrachloride carefully with a sprayer at a rate of about 3 liters/m²for about 30 seconds, the powder is rinsed on the lower sieve. When thesample powder has been rinsed completely, the upper sieve is removed andspraying all over the lower sieve is conducted for about four minutes.After that, the lower sieve is air-dried and a weight of the driedpowder remaining thereon is measured. A series of the above-mentionedsteps are repeated by using the 20 mesh sieve and each of the otherthree sieves of smaller meshes, respectively, and 5 g each of new samplepowder is used. In order to obtain an accumulated weight percentage, theweights of the powder remaining on each sieve are multiplied by 20 andthen those obtained values are plotted on a logarithmic probabilitypaper to openings of each sieve. Those plotted points are connected witha line, particle sizes corresponding to the accumulated weightpercentages 50(d₅₀) and 84(d₃₄) are read, and wet sieve size (d_(ws)) iscalculated by the following equation.${\log_{e}d_{ws}} = {{\log_{e}d_{50}} - {\frac{1}{2}\left( {\log_{e}\frac{d_{34}}{d_{50}}} \right)^{2}}}$

Flowability: Measured in accordance with the method described inJP-A-3-259925.

Namely, there is used a measuring apparatus comprising a support base42, an upper hopper 31 and a lower hopper 32. The both hoppers arealigned on their center lines and supported on the support base 42 asshown in FIG. 1 (corresponding to FIG. 3 described in JP-A-3-259925).The upper hopper 31 has an inlet 33 of 74 mm diameter, an outlet 34 of12 mm diameter and a partition plate 35. The height from the inlet 33 tothe outlet 34 is 123 mm. The partition plate 35 is provided on theoutlet 34, and thereby the powder in the hopper can be kept therein anddropped optionally. The lower hopper 32 has an inlet 35 of 76 mmdiameter, an outlet 37 of 12 mm diameter and a partition plate 38. Theheight from the inlet 35 to the outlet 37 is 120 mm, and the partitionplate 38 is provided on the outlet 37 like the upper hopper. The upperhopper and the lower hopper are adjusted so that the distance betweenthe both partition plates is 15 cm. In FIG. 1, numerals 39 and 40represent outlet covers of each hopper, and numeral 41 represents avessel for receiving the dropped powder.

Prior to measuring the flowability, about 200 g of powder is allowed tostand for not less than four hours in a room, the temperature of whichis adjusted to 23.5° to 24.5° C., and then sieved with a 10 mesh sieve(sieve opening: 1,680 μm). The measurement of the flowability is carriedout at the same temperature.

(I) At first, immediately after the upper hopper 31 is charged with justa cup of powder by using a 30 cc cup, the partition plate 35 is pulledout to drop the powder into the lower hopper. When the powder does notdrop, the powder is stuck with a wire. After the powder has droppedcompletely into the lower hopper 32, the dropped powder is allowed tostand for 15±2 seconds, and then the partition plate 38 of the lowerhopper is pulled out to see whether or not the powder is dropped fromthe outlet 37. When the powder is dropped completely within eightseconds, the powder is estimated to have been dropped as required.

(II) The same steps as above are repeated three times to see if thepowder is dropped as required. In case where the powder is droppedsatisfactorily twice or more, the flowability of the powder is estimatedto be “Good”. In case where the powder is never dropped, the flowabilityof the powder is estimated to be “Not good”. In case where in threeseries of the dropping test, the powder has been dropped only one time,the dropping test is further conducted twice, and when the two series ofthe dropping test are both satisfactory, the flowability is estimated tobe “Good”. In other cases, the flowability is estimated to be “Notgood”.

(III) With respect to the powder estimated to be “Good”, the upperhopper is charged with two cups of powder by using the same 30 cc cup,and the dropping test of the powder is conducted in the same manner asabove. When as a result, the flowability is estimated to be “Good”, thenumber of cups filled with the powder is increased successively and thedropping test is continued until the flowability is estimated to be “Notgood”. The dropping test is conducted up to eight cups at most. Thepowder having flowed out from the lower hopper in the previous droppingtest may be re-used.

(IV) The larger the amount of the PTFE powder is, the more difficult todrop.

The number of cups when the flowability is estimated to be “Not good” issubtracted by 1, and the obtained value is taken as “Flowability” of thepowder.

Average particle size and particle size distribution of granular powder:Standard sieves of 10, 20, 32, 48, 60 and 80 meshes (inch mesh) areplaced in that order from the top, and granular PTFE powder is put onthe 10 mesh sieve. The sieves are vibrated to drop smaller particlesdownward through each sieve in order. Then after the ratio of the powderremaining on each sieve is obtained by %, accumulated percentages(ordinate) of each remaining powder to the openings of each sieve(abscissa) are plotted on the logarithmic probability paper, and thosepoints are connected with a line. The particle size, the proportion ofwhich is 50% on that line, is obtained and is regarded as an averageparticle size. Also percents by weight of the granular powder remainingon each sieve of 10, 20, 32, 48, 60 and 80 meshes are regarded as theparticle size distribution.

Remaining amount of electrostatic charge-preventing compound: 50 Gramsof a granular PTFE powder is put in Vial bottle, and after the bottle issealed, the powder is heated at 200° C. for 60 minutes to give a sample.Then 5 cc of a sample gas is collected from the Vial bottle by means ofan injector, and an amount of the electrostatic charging-preventingcompound is determined by gas chromatography.

Electrostatic charge: Handy Electrostatic Meter SFM775 available fromIon Systems, Inc. is used to determine an electrostatic charge.

Tensile strength (hereinafter may be referred to as “TS”) and elongation(hereinafter may be referred to as “EL”): A die having an inner diameterof 100 mm is charged with 25 g of powder, and a pressure is appliedgradually over about 30 seconds until the final pressure becomes about300 kg/cm². Then that pressure is kept for two minutes to give apre-molded article. The pre-molded article is taken out of the die moldand put in an electric oven being kept at 365° C. to be subjected tosintering for three hours. Then the sintered article is punched with aJIS dumbbell No. 3 to give a sample. A stress at break and elongation ofthe sample are measured in accordance with JIS K 6891-5.8 by stretchingat a stretching rate of 200 mm/min with an autograph having a grossweight of 500 kg.

Whiteness (Z value): A 50 mm diameter die is filled with 200 g of agranular powder which is then maintained at a molding pressure of 300kg/cm² for five minutes. The obtained pre-molded article (diameter:about 50 mm, thickness: 50 mm) is heated up from room temperature to365° C. at a temperature raising rate of 50° C./hr. After having beenmaintained at 365° C. for 5.5 hours, the pre-molded article is cooled ata cooling rate of 50° C./hr and then the molded article is cut crosswisewith a lathe at the point about 25 mm from its end (center portion).Then the Z value on the center of the cut portion is measured through Zvalue measuring method of the XYZ colorimetric system of CommissionInternational de Leclairage.

EXAMPLES 1 to 4

A 200-liter granulation tank equipped with cone blades was charged with120 to 150 liters of ion-exchanged water, and the inside temperature ofthe tank was adjusted to 20° to 28° C. Then the tank was charged with 30kg of PTFE powder (POLYFLON TFE MOLDING POWDER M-12 available fromDAIKIN INDUSTRIES, LTD., TFE homopolymer) prepared by suspensionpolymerization and pulverized into an average particle size of 34 μm.The corn blades were rotated at 400 rpm, and two to three minutes after,19.1 kg of CH₂Cl₂ as an organic liquid was added in the tank.Subsequently the stirring was carried out at 400 rpm for five minutes tomake the organic liquid and PTFE powder compatible with each other.After that, a product in the granulation tank was passed through a linemixer outside the tank for external circulation for ten minutes. Theinside temperature of the tank was raised to 37° to 38° C. over 15minutes and kept at that temperature for 60 minutes to distil off theorganic liquid and give a granular PTFE powder. Physical properties ofthe obtained granular PTFE powder and physical properties of a moldedarticle obtained from the granular PTFE powder were determined by theabove-mentioned methods. The results are shown in Table 1.

The granular PTFE powder was subjected to the following if treatment forlowering its electrostatically charging property.

A 5% aqueous solution of surfactant was added to an aqueous dispersionof the above-mentioned granular PTFE powder in a concentration shown inTable 1 (concentration on the basis of the granular PTFE powder),followed by stirring for five minutes.

After the stirring was stopped, the granular powder was separated fromwater by using a 150 mesh sieve, and a collected granular powder wasdried at 165° C. for 20 hours by using a box type hot air circulatingdryer without washing with water to give the granular PTFE powder of thepresent invention which was subjected to treatment for lowering itselectrostatically charging property.

Physical properties of the obtained granular PTFE powder which wassubjected to the treatment for lowering its electrostatically chargingproperty and physical properties of a molded article obtained from thegranular PTFE powder were determined by the above-mentioned methods. Theresults are shown in Table 1.

Surfactants shown in Table 1 are those mentioned below.

DS-101: Ammonium perfluorooctanoate which is an anionic surfactantavailable from DAIKIN INDUSTRIES, LTD.

Puronon #208: A nonionic surfactant available from NOF Corporation andrepresented by the following formula:

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Before After Before After Before AfterBefore After treated treated treated treated treated treated treatedtreated Conditions for treating for lowering electrostatically chargingproperty Kind of surfactant — DS-101 — DS-101 — Puronon — Puronon #208#208 Concentration of surfactant — 0.05 — 0.1 — 0.025 — 0.05 (% byweight based on granular PTFE powder) Physical properties of granularpowder Apparent density (g/cm³) 0.83 0.83 0.82 0.82 0.85 0.85 0.83 0.83Flowability 8.0 7.0 8.0 8.0 8.0 8.0 8.0 8.0 Electrostatic charge (V)320.0 20.0 330.0 7.0 300.0 3.0 310.0 0.0 Particle size distribution (%by weight) 10 on 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20 on 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 32 on 4.0 4.4 5.0 5.4 6.6 6.7 4.1 4.0 48 on 54.4 54.748.2 48.0 53.3 53.4 50.1 50.2 60 on 21.8 20.5 17.8 17.6 16.5 16.2 18.018.1 80 on 16.2 15.6 23.5 23.1 17.5 17.4 20.1 19.9 80 pass 3.6 4.8 5.55.9 6.1 6.3 7.7 7.8 Average particle size (μm) 340 330 310 310 340 340310 310 Amount of remaining electrostatic — 57 — 110 — 62 — 70charge-preventing compound (ppm) Physical properties of molded articleTS (kg/cm²G) 430 390 400 400 390 390 400 400 EL (%) 400 380 390 390 390390 390 390 Whiteness (Z value) 110.1 110.1 110.0 110.0 111.0 111.0111.1 111.1

In the column of the particle size distribution of Table, 10 on, 20 on,32 on, 48 on, 60 on and 80 on indicate the percentages of particlesremaining on the 10 mesh, 20 mesh, 32 mesh, 48 mesh, 60 mesh and 80 meshsieves, respectively. And, 80 pass represents the percentage of theparticles passed through the 80 mesh sieve.

EXAMPLE 5

9.90 Kg (dry basis) of a PTFE powder (POLYFLON TFE MOLDING POWDER M-111available from DAIKIN INDUSTRIES, LTD.: modified PTFE) having an averageparticle size of 25 μm after pulverizing and 1.10 kg of a glass fiber(average diameter: 12 μm, average fiber length: 80 μm) subjected towater repellent treatment previously with an aminosilane coupling agentwere pre-mixed by using a 75-liter Henschel mixer.

A 10-liter granulation tank was charged with 6 liters of ion-exchangedwater and 2 kg of the above-mentioned mixture obtained by pre-mixing thePTFE powder and glass fiber. Thereto was added 1,200 ml of methylenechloride (organic liquid forming a liquid-liquid interface with water),followed by stirring at 800 rpm at 25° C.±2° C. for five minutes withcone blades for granulation.

Further the stirring was continued at 2,000 rpm for two minutes by usingdispersing blades of 100 mm diameter.

Then after shaping of the powder was carried out at 25° C.±2° C. for tenminutes under stirring at 800 rpm with cone blades, the insidetemperature of the tank was raised up to 38° C. over 20 minutes to givea granular PTFE powder. Physical properties of the obtained granularPTFE powder and physical properties of a molded article obtained fromthe granular PTFE powder were determined by the above-mentioned methods.The results are shown in Table 2.

The obtained granular PTFE powder was subjected to the followingtreatment for lowering its electrostatically charging property.

To the aqueous dispersion of the above-mentioned granular PTFE powderwas added a 5% aqueous solution of Puronon #208 which is a nonionicsurfactant, in a concentration shown in Table 1, followed by stirring at400 rpm for five minutes. After the stirring was stopped, the granularpowder was separated by using a 150 mesh sieve and the collectedgranular powder was dried at 165° C. for 16 hours with a box type hotair circulation dryer without washing with water, thus giving afiller-containing granular PTFE powder subjected to treatment forlowering electrostatically charging property.

Physical properties of the obtained granular PTFE powder subjected totreatment for lowering electrostatically charging property and physicalproperties of a molded article obtained from the granular PTFE powderwere determined by the above-mentioned methods. The results are shown inTable 2.

EXAMPLE 6

9.90 Kg (dry basis) of a modified PTFE powder (POLYFLON TFE MOLDINGPOWDER M-111 available from DAIKIN INDUSTRIES, LTD.) having an averageparticle size of 25 μm after pulverizing and 1.10 kg of a whollyaromatic polyester resin powder (ECONOL available from Sumitomo ChemicalIndustries, Ltd., finely pulverized powder, average particle size: 30 to40 μm) were pre-mixed by using a 75-liter Henschel mixer.

A 10-liter granulation tank was charged with 6 liters of ion-exchangedwater and 2 kg of the mixture obtained by the above-mentioned pre-mixingof the PTFE powder and ECONOL. Thereto was added 1,200 ml of methylenechloride (organic liquid forming a liquid-liquid interface with water),followed by stirring at 800 rpm at 25° C.±2° C. for five minutes withcone blades for granulation.

Further the stirring was continued at 2,000 rpm for two minutes by usingdispersing blades of 100 mm diameter.

Then after shaping of the powder was carried out at 25° C.±2° C. for tenminutes under stirring at 800 rpm with cone blades, the insidetemperature of the tank was raised up to 38° C. over 20 minutes to givea granular PTFE powder. Physical properties of the obtained granularPTFE powder and physical properties of a molded article obtained fromthe granular PTFE powder were determined by the above-mentioned methods.The results are shown in Table 2.

The obtained granular PTFE powder was subjected to the followingtreatment for lowering electrostatically charging property.

To the aqueous dispersion of the above-mentioned granular PTFE powderwas added a 5% aqueous solution of sodium tetradecenesulfonate (SOS)which is an anionic surfactant, in a concentration shown in Table 2,followed by stirring at 400 rpm for five minutes. After the stirring wasstopped, the granular powder was separated by using a 150 mesh sieve andthe collected granular powder was dried at 165° C. for 16 hours with abox type hot air circulation dryer without washing with water, thusgiving a filler-containing granular PTFE powder subjected to treatmentfor lowering electrostatically charging property.

Physical properties of the obtained granular PTFE powder subjected tothe treatment for lowering electrostatically charging property andphysical properties of a molded article obtained from the granular PTFEpowder were determined by the above-mentioned methods. The results areshown in Table 2.

TABLE 2 Ex.5 Ex.6 Before After Before After treated treated treatedtreated Conditions for treating for lowering electrostatically chargeproperty Kind of surfactant — Puronon — SOS #208 Concentration of —0.025 — 0.025 surfactant (% by weight based on granular PTFE powder)Characteristics of granular powder Apparent density (g/cm³) 0.76 0.760.70 0.70 Flowability 8.0 8.0 8.0 8.0 Electrostatic charge (V) 120.0 0.0220.0 0.0 Particle size distribution (%) 10 on 0.0 0.0 0.0 0.0 20 on 5.86.0 16.3 16.4 32 on 47.9 49.7 32.5 32.9 48 on 39.7 39.6 30.0 29.7 60 on5.1 3.4 9.7 9.6 80 on 1.1 1.0 8.0 7.8 80 pass 0.4 0.4 2.5 2.6 Averageparticle size (μm) 540 540 500 500 Remaining amount of — 60 — 68electrostatic charge- preventing compound (ppm) Physical properties ofmolded article TS (kgf/cm²G) 160 160 160 1190 EL (%) 328 328 300 3Whiteness (Z value) 96.7 96.7 — 111.1

INDUSTRIAL APPLICABILITY

According to the present invention, electrostatic charging of thegranular powder can be inhibited even after drying, and good powderflowability can be maintained at the time of transportation and molding.

What is claimed is:
 1. A process for preparing alow-electrostatically-charging granular polytetrafluoroethylene powderprepared by contacting a nonionic surfactant having an electrostaticcharging-preventing ability when substantially dry to a granularpolytetrafluoroethylene powder, and then drying the granular powderwithout washing while the nonionic surfactant is kept remaining in thepowder, said nonionic surfactant is contacted in the form of an aqueoussolution at a concentration of not more than 0.05% by weight to thegranular polytetrafluoroethylene powder.
 2. The preparation processclaim 1, wherein the granular polytetrafluoroethylene powder does notcontain a filler.
 3. The preparation process of claim 1, wherein thegranular polytetrafluoroethylene powder contains an electricallyinsulating filler.
 4. A granular polytetrafluoroethylene powder whichcontains a segmented polyalkylene glycol in an amount of 10 to 70 ppmand has an electrostatic charge of not more than 10 V.
 5. The granularpolytetrafluoroethylene powder of claim 4, wherein the granularpolytetrafluoroethylene powder does not contain a filler.
 6. Thegranular polytetrafluoroethylene powder of claim 4, wherein the granularpolytetrafluoroethylene powder contains an electrically insulatingfiller.